Which feature will not transfer packets when there is silence?

VAD
As we listen and pause between sentences, typical voice conversations can contain up to 60 percent silence in each direction. In circuit-switched telephone networks, all voice calls use fixed bandwidth 64-kbps links regardless of how much of the conversation is speech and how much is silence.

In multiservice networks, all conversation and silence is packetized. Using VAD, you can suppress packets of silence. Silence suppression at the source IP telephone or VoIP gateway increases the number of calls or data volumes that can be carried over the links, more effectively utilizing network bandwidth. Bandwidth savings are at least 35 percent in conservative estimates.

VAD is enabled by default for all VoIP calls. In real-world practice, is it suggested that VAD be avoided because it creates quality issues and breaks applications such as fax and modem transmissions.

For G.729 bandwidth is reduced from 26.4 kbps to 17.2 kbps with VAD and to 7.3 kbps with VAD and cRTP enabled.

Cisco Press CCDA 640-864 Official Certification Guide Fourth Edition, Chapter 14